FCC is a major secondary refining process and the FCC catalyst used in such a process provides considerable flexibility to refiners to meet the required product slates. To improve the efficiency of this process it is desired to upgrade the bottom products (boiling above 370.degree. C.) to more valuable lighter products and less coke (due to regenerator hardware limitations). In this endeavor, not much emphasis is given in the prior art processes of FCC catalyst development.
Preparation of FCC catalyst is generally accomplished by admixing Y type zeolite, normal kaolin clay and suitable binders to produce a slurry. The aqueous slurry is then subjected to agitation for uniform dispersion of individual components and spray dried to form dry catalyst microspheres.
Typical prior art catalyst preparation procedure involves use of sodium silicate as silica source, aluminum sulfate, Y type zeolite, kaolin clay and suitable acid to convert sodium silicate to silica sol binder. However, such a process involves repeated washing off of sodium sulfate by product and subsequent exchange of sodium of the zeolite. This process is time consuming, hardware intensive and requires considerable quantities of demineralised (DM) Water and there is a need for effluent treatment facility. U.S. Pat. No. 4,142,995 mentions use of silica-alumina gel in catalyst formulation for improving catalyst activity and attrition resistance. However, this process has not addressed the issue of selectivity improvements like reduction of bottom and coke and enhancement of TCO and gasoline yields. It is known that preparation of silica-alumina gel is time consuming and requires several washings.
From an efficiency and economy point of view it is generally preferred to use ingredients with low soda content i.e., binder, zeolite, matrix etc. to avoid series of washing steps. U.S. Pat. No. 4,443,553 teaches preparation of FCC catalyst using soda free raw materials, wherein aluminum hydroxy chloride is used as viscosity reducing agent. The process suffers from the disadvantage that chlorine evolved during the preparation procedure is corrosive and poses problems for the hardware and environment as well. Performance of the catalyst is not part of claims of the said patent. In addition to the above, presence of chlorine is likely to interfere with the efficiency of catalytic cracking.
U.S. Pat. No. 4,086,187 describes use of ammonium polysilicate and alumina to provide catalyst with improved attrition resistance and pore volume. However the catalyst has not been subjected to detailed performance testing.
While the prior art describes different methods of formulating cracking catalyst often requiring high investments, catalysts are not able to meet the desired product requirements. Thus, there is a requirement for a catalyst which provides not only an acceptable degree of physical properties and activity but also the most desirable selectivity like enhanced gasoline and TCO yields while reducing undesirable bottom and coke products. Development of bottom and coke selective catalyst becomes more relevant for low severity FCC unit operations prevalent in many countries like India.
Prior art processes demonstrate the performance of the steamed catalyst by means of ASTM Micro Activity Test (MAT), where the feed injection time is very large leading to highly non-isothermal reaction conditions. The feed used is different than that used in commercial FCC units and only a particular catalyst to oil ratio is employed. The product selectivity depends on these parameters and ASTM MAT is not suitable for predicting performance of the catalyst in a commercial FCC unit. To overcome this difficulty and to correctly predict catalyst performance in a commercial FCC unit, in the present invention a commercial high vacuum gas oil feedstock is used to evaluate the catalyst at different catalyst to oil ratios at contact times much lower than the conventional ASTM MAT conditions.